THE PROTEIN PHOSPHATASES RESPONSIBLE FOR DEPHOSPHORYLATION OF HORMONE-SENSITIVE LIPASE IN ISOLATED RAT ADIPOCYTES

The levels of the cytosolic serine/threonine protein phosphatases (PP) in rat adipocyte extracts have been determined, by using both referen ce substrates and hormone-sensitive lipase (HSL) as substrates. Adipoc ytes contain significant levels of both PPI and 2A (1.6 and 2.0 m-unit s/ml of packed cells respectively), with lower levels of PP2C and virt ually no PP2B activity. PP2A and 2C exhibit similar degrees of activit y against HSL phosphorylated at site 1, together accounting for 92% of the total. In contrast, site 2 is dephosphorylated predominantly by P P2A (over 50% of total activity), whereas PP1 and PP2C contribute appr ox. 20% and 30% respectively to the total phosphatase activity against that site. Total phosphatase activity in the adipocyte extracts was 2 -3-fold higher against site 2 than against site 1. The possible signif icance of these findings to the regulation of HSL activity in adipose tissue in vivo is discussed.